Feed materials that are commercially manufactured for domestic animals such as fish, dogs, cats, poultry and swine frequently contain the maximum amount of lipid that can be incorporated into the materials without altering their desired physical form. This is done for purposes such as lowering the cost of the materials, maximizing animal performance results, and minimizing the amount of excrement voided by the animals.
In the case of fish farming, compacted feed materials commonly consist of feed pellets of standard size and uniform composition, for ease of administration to and diet control of the fish. The lipid ingredients are typically of marine origin, such as fish oils. Standard fish feed pellets commonly contain 10-18% by weight of fish oil. Increases in this level, desirable to increase the nutritional value of the feed per unit weight, adversely affect the structural integrity of the pellets, in terms of hardness and surface oiliness. Then the pellets tend to lose their free-flowing nature during transportation.
The commonly adopted process for making fish feed pellets is one of mixing the ingredients and then pelletizing them. Pelletizing is essentially a process of compaction. The pellets are compressed to certain hardness, porosity and density over which there is little control or flexibility. Most of the oil content must be included in the mixture prior to pelletizing, but attempts to include larger amounts than about 10-18% lead to the formation of pellets of insufficient hardness and durability. Whilst small amounts of additional oil can be incorporated by adding oil subsequently to the pellets, this is a time-consuming step. The subsequently added oil only attaches to and becomes absorbed in the pellets slowly, and for the most part remains on the surface, as it cures, over about 48 hours. Before that process is completed, the pellets cannot acceptably be bagged and shipped.
Some pelletized fish feeds and other pelletized pet foods are conventionally made by an extrusion method. In the extruder, the ingredients may be mixed, cooked, sheared, gelatinized, and formed and chopped into pellets, rather than simply compacted and chopped into pellets in a pelletizing process. The extrusion process allows more control over density and porosity of the product, than in the case of pelletizing, although it is a more expensive process and requires more expensive equipment. For example, the control can be exercised by varying the degree of cooking, or by varying the extruder outlet conditions to provide for greater expansion of the product on issue from the extruder, so as to form a less dense product. Additional oil can be added to an extruded pelletized product, but again the amount is limited by surface effects.
Other pet food products which are handled in pellet form include textured vegetable proteins such as textured soya protein (TSP), which is basically a fully cooked defatted soya protein. This is used as a meat extender in canned dog foods and cat foods. TSP is prepared by cooking and extruding the material, then dehydrating it in pellet-like form for storage and transportation. It is re-hydrated prior to addition to the food cans. If its density after re-hydration is not correct, it will not mix adequately with the other ingredients.
Further, various pet foods for dogs and cats are produced in hard pellet forms, and contain varying quantities of lipids. Poultry feeds and swine feeds similarly are prepared and used in pellet form. In all these cases, it is often desirable that the pellets be of uniform, controlled composition, with high lipid content.
It is an object of the present invention to provide a novel process for preparing animal feed pellets.
It is a further object of the invention to provide such a process which can yield feed pellets of improved nutritional content and controlled density and texture.